Related fields include foils that vary the inherent fluid-dynamic characteristics of attached vessel hulls or vehicle bodies by deriving lift, altering trim, reducing turbulence, or diminishing wave resistance; in particular, removable fins and foils for surfboards, sailboards, and pleasure or sport vessels.
Foils, fins, centerboards, skegs, and hydrodynamic keels can optimize lift, reduce drag, and stabilize a vessel hull or vehicle body (hereinafter generically referred to as “hulls”) when mounted at a suitable angle of attack relative to the oncoming fluid. Foils also enhance drive and maneuverability in the fluid, acting in some ways like the friction of wheels on a solid surface. Here, a “fluid” may be either water or air.
Foil mountings can be permanent or removable. For example, a foil can be permanently attached with fasteners or adhesive to an inner layer partially constructed hull, then both foil and hull can be overcoated or overwrapped with outer layers such as fiberglass and resin. Removable foils can be inserted and withdrawn from sockets built into the hull, and fixed in place with fasteners or the like. Removable foils allow a user to change the number, type, or position of foils, adapting the hull to different fluid-dynamic performance demands imposed either by different surrounding conditions, different users, or different types of use. They also allow a user to quickly replace a damaged foil, or remove foils for easier storage or transport.
These removal and replacement operations can take place on beaches and shores, or sometimes even in the water. In these settings, mountings that require few or no tools to remove and replace a foil are convenient. Many users will sacrifice convenience if the foils are likely to wholly or partially self-detach in strong or turbulent currents. On the other hand, some foils are intentionally made frangible so that a substantial impact causes them to break away rather than damage the hull or injure the user or a nearby person or animal.
Rigid mountings and rigid foils can be simple and rugged, but they affect maneuverability when rapid turns are desirable or when the flow direction of the ambient fluid may change suddenly. The fluid-pressure drop causes bubble formation in the turbulent water on the leeward side of the foil, and the resulting cavitation or “crabbing” increases drag and can cause loss of control of the board or other craft; if the foil leaves the water completely, stall occurs, and there is a loss of control similar to a land vehicle's hydroplaning in a water puddle. Multiple foils mounted rigidly at different angles aid in turning, but can create drag that reduces the speed during carving into the face of the wave or straight-line travel. In a curve, a fin angled at a non-optimal orientation (not tangent to the curve) also increases drag. In addition, water leakage or shocks associated with repeated torsional stresses over time may cause strain damage to the foil mounting, or hull.
Self-aligning foils have a yielding component so that they tilt, pivot or flex temporarily when the angle of attack changes rapidly, then return to a default orientation and position as the surrounding forces stabilize. The angle of attack can be changed intentionally (as when turning) or a change can be imposed by external factors (as when an ambient current changes direction). A self-aligning foil can boost a vessel's or vehicle's acceleration through a curve and improve control in turbulent fluid environment. On many watercraft including surfboards and other aquatic-sports boards, most of the hydrodynamic force from the change of direction is concentrated on the leading edge of the foil. The pivoting range of the fin should not be too wide or too free, lest it delay the return to equilibrium after the turn or destabilize the board.
Therefore, vessels or vehicles with removable foils would benefit from a mounting system with an internal resilience property that would cause the foil to yaw or roll independent of the hull when the fluid-pressure difference between one side of the foil and the other exceeds a predetermined threshold, then return to a default orientation when the fluid-pressure difference falls below the threshold. Preferably, the yaw or roll angle would be limited to a useful range for the application. Preferably, some embodiments would allow the leading edge of the foil, where the pressure differences can be highest, to move. Preferably, the mounting would distribute torsional stresses and absorb shocks to lengthen component life. Preferably, the mounting and dismounting of foils would require no tools, but its configuration should reduce the probability of accidental detachment during use. Preferably, embodiments for sailboards or sailboats would include a compatible translation adjustment. Preferably, some embodiments of the mounting would be easy to install at non-perpendicular angles if desired. Preferably, embodiments of the mounting could be durably secured to foam layers of certain hulls.